1. Field of the Invention
The present invention relates in general to magnetic resonance tomography, or MRT as is used in the field of medicine in order to examine patients. In particular, the present invention relates to a method for the magnetic resonance tomographic representation of vessels (angiography) without the use of contrast agent, and without physiological synchronization with the execution of the MR sequence.
2. Description of the Prior Art
The term “angiography” refers in general to the representation of blood vessels (arteries and/or veins) using medical imaging methods (x-ray, CT, MRT). In magnetic resonance angiography (MRA) there are two classical methods that make use of the property of the flow of blood in order to represent the vessels (time of flight methods, or TOF, and phase-contrast angiography, or PC angio). There is also MRA supported by contrast agent, which uses contrast agent that increases spin relaxation in order to obtain a signal-rich representation.
Time of flight MRA exploits the flow of blood into the imaging volume for angiographic representation.
In this technique, the blood flowing in is completely relaxed and emits, as an endogenous (i.e., not supplied to the body) contrast agent, a strong signal. As a consequence of a rapid sequence of RF pulses, the stationary tissue experiences a strong saturation, and ultimately supplies only a small signal contribution. The TOF measurement is preferably carried out in the time domain using a gradient echo sequence. The asymmetrical excitation form of this sequence brings about a comparable cross-magnetization of the inflowing blood in the overall 3D volume. TOF technology is used both as a 2D method and as a 3D method. The 2D version is preferably used in the cervical spine region, because there, due to the high speed of blood flow, the blood is completely exchanged within a repetition time span TR. In this region, it is therefore possible to achieve a very high degree of magnetization for the imaging using a large excitation pulse angle. A disadvantage of the TOF method is its sensitivity to patient movement, in particular in the cervical region.
Another method is phase-contrast angiography method. Like TOF angiography, phase-contrast angiography (PC-angio or PCA) uses the blood flow for the selective representation of the vessels in the MRT.
In TOF, a bipolar gradient is used for first-order flow compensation. Conversely, in PCA a bipolar gradient Gb is used for the coding of the flow speed in order to generate an additional phase proportional to the speed vx:ΔΦvx=γ·Gb·Vx·τ2,where τ is the duration of Gb. The complex subtraction of an image that is flow-compensated in the x direction from a flow-sensitive image accordingly results in an image whose pixel vectors have a magnitude and a phase proportional to the speed vx. It follows from this that the stationary spins do not supply any contribution.
Sequences having different phase sensitivity therefore allow, after subtraction, a background-free representation of vessels. A disadvantage of this method is that it can be applied only to one particular speed interval at a time, in order to avoid ambiguities.
In recent years, contrast agent-supported angiography (CE-MRI) has become widely used for almost all bodily regions. It permits both dynamic and static examinations to be made in a very short measurement time. The functioning of contrast agents in MR is based in general on an influencing of some of the parameters T1, T2 that are decisive for the contrast, with the aid of atoms or molecules that have a sufficiently large magnetic moment (e.g. gadolinium Gd3+). However, as free ions all these substances are highly toxic, and thus cannot be used in this form. The toxicity (poisonousness) can be reduced by binding these ions in what are known as chelate complexes. However, recently there have been increased reports of (kidney) ailments (Nephrogenic Systemic Fibrosis, or NSF) caused by contrast agents containing gadolinium. Therefore, recently work has been done on methods that do not make use of contrast agent, i.e. that are contrast agent-free.
A method recently developed by Toshiba is called fresh blood imaging (FBI). It essentially makes use of the fact that the T2 relaxation time of blood is much longer than that of the stationary tissue that surrounds the blood vessel system. The FBI imaging sequence is based in principle on a (EKG- and respiration-triggered) Toshiba-specific FASE (Fast Advanced Spin Echo) sequence.
Another method, developed by Siemens, is known as SPACE, and also uses EKG triggering. SPACE is a variant of the 3D-turbo spin echo technique. In comparison with the conventional turbo spin echo sequence, SPACE uses non-selective, long refocusing pulse trains that are made up of RF pulses having variable flip angles. This permits very high turbo factors (measurement time advantage of a turbo SE sequence compared with a conventional spin echo sequence: >100) and a high scanning efficiency. This results in high-resolution isotropic images that allow free reformatting in all planes. In addition, EKG triggering enables angiography with large field of views (FOVs), as is required for example in the context of peripheral angiography of the legs. Nonetheless, FBI and SPACE represent very complicated measurement methods, because a triggering, in particular triggering by breath and EKG, as is required both in FBI and in SPACE, is generally very expensive:
A respiration monitor belt has to be applied, and a number of EKG electrodes must be affixed to the body. The sequence supplies only data that fit a corresponding cardiorespiratory rhythm and the respiratory and heartbeat measurement devices have to be combined and configured in terms of measurement technology with the MRT measurement system (installation computer, sequence controlling).